Piezoelectric ceramic materials currently in practical and widespread use are lead-containing piezoelectric ceramic materials typified by lead titanate (hereinafter simply referred to as “PT”) and lead zirconate titanate (hereinafter simply referred to as “PZT”). The lead-containing piezoelectric ceramic materials contain lead (Pb) components and thus have the problem of environmental effects during productions and during and after uses. There is a demand for the urgent development of lead-free piezoelectric ceramic materials.
Further, the piezoelectric ceramic materials have Curie points and lose their piezoelectric properties in temperature ranges exceeding the Curie points. In general, the Curie points of the lead-containing piezoelectric ceramic materials are on the order of 200 to 500° C. There is a demand for piezoelectric ceramic materials usable at higher temperatures.
As a piezoelectric ceramic material that satisfies the above demands, a bismuth layer-structured ferroelectric “Na0.5Bi4.5Ti4O15 (hereinafter occasionally simply referred to as “NBT”)” is known. The NBT has a high Curie point of about 670° C., which is higher than the Curie points of the PT and of the PZT, and is thus expected as a lead-free piezoelectric ceramic material usable in a high temperature range over 500° C. Each of Patent Documents 1 to 3 and Non-Patent Documents 1 and 2 discloses a piezoelectric ceramic composition containing the NBT as a main component.
Patent Document 1: Japanese Laid-Open Patent Publication No. 50-67492
Patent Document 2: Japanese Laid-Open Patent Publication No. 11-29356
Patent Document 3: Japanese Laid-Open Patent Publication No. 2007-119269
Non-Patent Document 1: “Piezoelectricity in Ceramics of Ferroelectric Bismuth Compound with Layer Structure”, S. Ikegami and I. Ueda, Japanese Journal of Applied Physics, 13 (1974), p. 1572-1577
Non-Patent Document 2: “Grain-Oriented and Mn-Doped (NaBi)(1-x)/2CaxBi4Ti4O15 Ceramics for Piezo- and Pyrosensor Materials”, T. Takenaka and K. Sakata, Sensor and Materials, 1 (1988), p. 35-46